As is known, squirrel-cage rotors are used in three-phase asynchronous motors, which are highly reliable and inexpensive drives and are also used as alternators in some specific applications.
In the standard configuration for high-power and medium electric voltage applications, for example in railway traction drives, the squirrel-cage rotor comprises a lamination stack made of magnetic material, with a series of slots engaged by respective bars made of aluminium or copper. The bars are parallel to one another, and protrude from the lamination stack at both axial ends of the rotor in a substantially symmetrical manner. The apexes of the bars are welded to two shorting rings, which define the axial ends of the rotor.
The torque moment of an asynchronous motor is generated by the interaction between the magnetic induction field in the air gap, produced by a symmetrical system of currents flowing through the stator windings, and the magnetic induction field in the air gap, generated by the induced currents flowing in the bars of the rotor. The torque moment of the bars is transferred to the lamination stack by the force or pressure that the bars exert on the surfaces of the slots in a tangential direction.
When the stator of an asynchronous motor is supplied by an inverter, the currents flowing through the windings of the stator and rotor are not perfectly sinusoidal, but have harmonics that are not negligible. Stator current harmonics that exceed the fundamental frequency generate air gap magnetic field harmonics, sufficient to induce harmonic currents in the rotor bars.
The interaction between the harmonics at different frequencies of the stator magnetic induction field and the rotor magnetic induction field causes torque moment oscillations.
The frequencies of the torque moment oscillations are given by the sum of or difference between the frequencies of the stator and rotor magnetic fields. Thus, the frequencies of the torque moment oscillations depend on the fundamental frequency of the alternating current set by the inverter in the stator windings, and on the inverter switching frequency. In electric traction drives the alternating current supplied to the stator must have a variable main frequency, so that the motor can run at different speeds. Thus, the range or spectrum of the frequency of said torque moment oscillations is extremely wide in such applications.
The lamination stack and the shorting rings have a different rotational inertia, which means that the torque moment oscillations cause deformations in torsion with an oscillating amplitude, in the end portions of the bars that are arranged between the shorting rings and the lamination stack.
Resonance occurs when the frequency of the torque moment oscillations is close to the torsional eigenfrequency of some components of the rotor and/or of the system driven by the motor. Due to said resonance, the extent of the deformations at the end portions of the bars is significant, and faults often occur due to fatigue breakages in the areas of the joints between the bars and the shorting rings, especially in electric railway traction applications, where highly dynamic and high-power performance levels are required.
The length of the end portions of the bars that protrude with respect to the lamination stack could be reduced to diminish the deformations due to the torque moment oscillations. However, this solution is not satisfactory, as such end portions must in any case be of a minimum length, i.e. there must be a minimum axial distance between the lamination stack and the shorting rings, to guarantee sufficient ventilation for cooling.
Another solution for reducing the deformations due to the torque moment oscillations, proposed by Siemens, consists of bending the end portions of the bars according to different configurations, for example leaving one bar straight and bending the two adjacent bars in order to move their apexes closer to that of the straight bar. Siemens recently also proposed a rotor with no shorting rings, in which it replaced the traditional bars with flexible conducting wires connected to one another.